I may be mistaken, but would it require the consensus of everyone who uses Bitcoin (as opposed to only those who mine) to enact such a change? I am still fairly new to how the Bitcoin protocol is designed.
Celroc
Not everyone. If you have more than 51% of all computing power - you own blockchain
If some percentage of miners decide to continue with the 25 BTC subsidy forever then on the block that the protocol dictates that the subsidy should drop to 12.5 BTC there will be a hard fork of the blockchain. There will be
two blockchains: one with blocks which have the correct 12.5 BTC subsidies and the other one containing the invalid blocks with 25 BTC subsidies.
If you do not change any of the network nodes then all blocks with the wrong subsidy will be dropped - end of story. So, even if 90% of the miners decide to continue the 25 BTC subsidy all of their blocks will be dropped unless they also convince some of the nodes to change also.
So on that block, assuming some percentage of miners and nodes (and wallets) decide to change the protocol
a new alt coin is born. It is just another in the growing number of alts. Bitcoin will just keep chugging along and will compete with the new alt in the marketplace.
I hope that answers your questions.
More technical details can be found here if you are interested:
https://bitcointalk.org/index.php?topic=352734.0